(a) Field of the Invention
The present invention relates to an image transmission optical system for an endoscope and, more particularly, to an image transmission optical system to be used with an inflexible-type endoscope or the like which is arranged to transmit an image by using a plural number of relay lenses.
(b) Description of the Prior Art
An inflexible-type endoscope arranged to transmit an image by using a plural number of relay lenses is composed that an objective 2, image transmission optical systems 3, 3', . . . and eyepiece 4 are arranged in an outer tube 1 in the order from the object side as shown in FIG. 17. The above-mentioned type of endoscope is arranged that an image Q of an object formed by the objective 2 is relayed in turn as Q.sub.1, Q.sub.2, . . . by means of respective image transmission optical systems 3, 3', . . . , and the last image Q.sub.L is observed through the eyepiece 4. The image transmission optical system disclosed in Japanese publised examined patent application No. 5993/74 is known as an image transmission optical system to be used with the above-mentioned type of endoscope. Said known image transmission optical system is arranged that two rod-like cemented doublets, each consisting of a positive lens element and negative lens element cemented together, are arranged so that the negative lens elements thereof face each other as shown in FIG. 18.
In case of said known image transmission optical system, spherical aberration, coma and chromatic aberration are corrected favourably. However, curvature of field is not corrected satisfactorily and, moreover, astigmatic difference is caused. Therefore, when the number of image transmission optical systems constituting the endoscope becomes large, curvature of field and astigmatic difference are accumulated and become very large. As a result, it is impossible to bring both of the central portion and marginal portion of the image into focus at the same time.
The image transmission optical system disclosed in Japanese published unexamined patent application No. 4245/77 is known as an image transmission optical system arranged to eliminate the above-mentioned disadvantage. Said known image transmission optical system is composed that two rod-like meniscus lenses 7 and 8, which are arranged so that the concave surfaces thereof face each other, are arranged between two rod-like biconvex lenses 5 and 6 as shown in FIG. 19 and is arranged that curvature of field and astigmatic difference are corrected by means of negative action of an air lens 9 which is formed by those two concave surfaces.
However, also in case of said known image transmission optical system, the image surface somewhat inclines toward the negative side. Therefore, when the number of image transmission optical systems to be used for transmission of image is large, curvature of field is accumulated and, as a result, flatness of image becomes unfavourable.